Photosynthetic culture apparatus and group of photosynthesis culture apparatuses

ABSTRACT

A photosynthetic culture system has a culture bath holding an fluid containing plant microorganisms, carbon dioxide supplying means for supplying carbon dioxide to the fluid in the culture bath, light-conducting plate in the form of a flat plate placed oppositely to a light-receiving culture surface existing on the side of said culture bath, and light-receiving panel mounted on the upper end surface of the light-conducting plate. Further said light-conducting plate has the function of turning incident light from said light-receiving panel by right angles to conduct the light uniformly to said light-receiving culture surface of said culture bath.

THIS APPLICATION IS A U.S. NATIONAL PHASE APPLICATION OF PCTINTERNATIONAL APPLICATION PCT/JP98/04575.

TECHNICAL FIELD

This invention relates to a photosynthetic culture system which fixescarbon dioxide by photosynthetically culturing and growing algae, plantmicroorganisms or the like.

BACKGROUND ART

Fossil fuels such as coal, oil and natural gas recently used in thermalpower plants etc. release a vast amount of carbon dioxide into theatmosphere by burning. Increase of the released carbon dioxide in theatmosphere will deteriorate global environment by causing global warmingetc., in addition, it will significantly affect the human societythrough the occurrence of natural disaster and heavy damage of crops,because increase of carbon dioxide in the atmosphere is the cause of afrequent drought, heavy rain and floods.

Thus, the development of the technology has been long-awaited which canfix carbon dioxide with less energy to decrease the amount of carbondioxide released into the atmosphere. And as a simple, safe andeffective method, a method has been investigated and is about to beutilized to fix carbon dioxide in the atmosphere using photosynthesis ofplant microorganisms which is caused by irradiation of sunlight etc.However, in the construction of the conventional photosynthetic culturesystems using plant microorganisms, since their culture baths need toensure a certain volume of fluid, after photosynthesis proceeds to someextent, light required for photosynthesis does not reach the wholesolution sufficiently. Therefore, methods have been used to makephotosynthetic culture baths shallow or stir the fluid in the bath sothat light will reach the whole solution.

For example, the amount of carbon dioxide in burning gas released fromthermal power plants is as vast as about 5000 t/day at 500000 kW outputpower of burning natural gas, and when burning coal or oil, carbondioxide emission will be further increased. In such a situation, inorder to fix carbon dioxide released from thermal power plants, thesystems which are as compact as possible and need less energy for thefixation are desirable. In order to solve these problems, improvementsare required in the carbon dioxide fixing technology which are now inuse globally such as enhancement of the carbon dioxide fixing ability ofphotosynthetic culture baths and systems including an optical system,enhancement of efficiency, increase of controllability of the productsof photosynthetic reaction.

However, in view of the fact that an effective depth of the culture bathfor photosynthesis is only several cm, one of the above methods, inwhich photosynthetic culture baths are made shallow, has a problem thatthe area of the photosynthetic culture systems must be extremelylarge-scale in order to ensure sufficient volume of culture solution.And the other method, in which the fluid is stirred, has also a problemthat the whole culture bath is not made good use of since the fluidalways subjected to photosynthesis is restricted within the limits wherelight can reach.

Further, in the conventional photosynthetic culture baths utilizingnatural sunlight, since the surface of the solution which is alight-receiving surface is irradiated with an intensive sunlight, lightintensity more than needed is wasted and it causes some troubles. Inaddition, with the progress of photosynthesis, the number of the cellsis increased, which prevents light from reaching the depths of thefluid.

DISCLOSURE OF THE INVENTION

In light of these problems of the prior art, the purpose of the presentinvention is to provide a photosynthetic culture system and a collectivephotosynthetic culture system which allow to control the waste ofoptical energy, to make good use of the whole culture bath, and tocontrol the increase of their installation area even when ensuring asufficient volume of culture solution.

In order to cope with the foregoing problems and subjects, the presentapplicants propose the present invention, based on the basic andscientific study on photosynthetic media, the discovery of thephotosynthetic media having good carbon dioxide fixing ability, a goodknowledge of operation in a thermal power plant and a deep knowledge ofcarbon dioxide emission and its fixation, which will realize theintroduction of photosynthetic media having the good carbon dioxidefixing ability as well as the introduction of the optimum environment topromote efficiency of photosynthesis and has the features describedbelow.

The photosynthetic medium used in the present invention is, for example,Euglena gracilis which are plant microorganisms.

With the progress of photosynthesis, the number of the photosynthesizedcells is increased, which prevents light from reaching the depths of thefluid. Therefore, the thickness of the culture solution is decreased inthe direction in which light travels. In addition, in order to eliminatethe waste of energy due to an excess of the natural sunlight irradiationover the required amount, the area of the natural sunlight-receivingsurface is effectively expanded and the light is conducted to thelight-receiving culture surface via the natural sunlight-receivingsurface. The natural sunlight-receiving surface and the light-receivingculture surface are arranged to have a rectangular relationship.

The present invention according to one aspect is a photosyntheticculture system comprising: a culture bath holding an fluid containingplant microorganisms, carbon dioxide supplying means for supplyingcarbon dioxide to the fluid in the culture bath, light-conducting meansin the form of a flat plate placed oppositely to a light-receivingculture surface existing on the side of the culture bath, andlight-receiving means mounted on the upper end surface of thelight-conducting means, wherein the light-conducting means has thefunction of turning the incident light from the light-receiving means bysubstantially right angles to conduct the light uniformly to thelight-receiving culture surface of the culture bath.

The present invention according to another aspect is a collectivephotosynthetic culture system, wherein more than one photosyntheticculture system according to are arranged so that the light-receivingculture surfaces of said culture baths will be in parallel to oneanother and the photosynthetic culture systems are connected to oneanother with a connecting pipe for supplying an fluid, a connecting pipefor transferring products, and a connecting pipe for supplying carbondioxide.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating the construction of one example of aphotosynthetic culture system made pursuant to Embodiment 1 of thepresent invention.

FIG. 2 is a diagram illustrating the construction of another example ofa photosynthetic culture system made pursuant to Embodiment 1 of thepresent invention.

FIG. 3 is a schematic diagram illustrating the construction of acollective photosynthetic culture system made up by laminating more thanone photosynthetic culture system, as a unit, made pursuant toEmbodiment 1 of the present invention.

FIG. 4 is a diagram illustrating the construction of a culture bath anda light-conducting portion of a photosynthetic culture system madepursuant to Embodiment 2 of the present invention.

FIG. 5(a) is a diagram illustrating the construction of a photosyntheticculture system made pursuant to Embodiment 2 of the present invention,and FIG. 5(b) is a diagram illustrating another example of a condenser.

FIG. 6 is a diagram illustrating the construction of a collectivephotosynthetic culture system which is made up by laminating more thanone photosynthetic culture system, as a unit, made pursuant toEmbodiment 2 of the present invention.

FIG. 7 is a diagram illustrating one example of a photosynthetic culturesystem of the present invention which uses a light-conducting plate inwhich incident light is diffused on both its surfaces.

FIG. 8 is a diagram illustrating one example of a photosynthetic culturesystem of the present invention in which the convex surface of thecondenser is turned downwardly.

DESCRIPTION OF SYMBOLS

1 Plant Microrganisms

3 Culture Bath

4 light-Receiving Culture Surface

5 Carbon Dioxide-Supplying Means

8 Light-conducting Plate

10 Light-Diffusing Surface

12 Light-Converging Portion

13 External Light-Receiving Surface

16 Photosynthetic Culture System

17 Connecting Pipe for Supplying Fluid

18 Connecting Pipe for Transferring Product

19 Connecting Pipe for Supplying Carbon Dioxide

22 Condenser

31 Semicylindrical Lens

32 Integrated Lens

BEST MODE OF THE EMBODIMENTS

Referring now to the embodiments of the present invention with referenceto the accompanying drawings.

Embodiment 1

FIG. 1 is a diagram illustrating the construction of one example of aphotosynthetic culture system made pursuant to Embodiment 1 of thepresent invention. Specifically, the photosynthetic culture system madepursuant to Embodiment 1 of the present invention basically comprises: aculture bath 3 which holds an fluid 2 containing plant microorganisms 1and whose side consists of a light-receiving culture surface 4; carbondioxide-supplying means 5 which supplies carbon dioxide to the fluid 2in the culture bath 3; a light-conducting plate 8 in the form of a flatplate (or a light-conducting cavity), as light-conducting means, whichhas a light-converging portion 12, as light-receiving means, having onits top a flat external light-receiving surface 13 receiving light 14from outside and which has a light-intake 9 for taking in the lightconverged in the light-converging portion 12 and which turns theincident light nearly right angles and reflects the light 15 to thelight-receiving culture surface 4 of the above culture bath 3. Here, oneof the surfaces of the light-conducting plate 8, which is on the side ofthe culture bath 3, consists of a diffusing surface 10 provided with,for example, a light-diffusing sheet which diffuses light, and the othersurface is provided with a diffused reflection layer 11. And the culturebath 3 is connected to an fluid-supplying pipe 6 for supplying an fluidand a product-taking pipe 7 for taking products. Materials for thelight-converging portion may be those having a high light transmittancy,such as acrylic and glass, and they can be selected in terms of cost,performance, machinability etc.

In the above construction, external light 14 enters through the externallight-receiving surface 13 and is converged in the light-convergingportion 12, and the converged light enters through the light-intake 9 ofthe light-conducting plate 8. The light having entered thelight-conducting plate 8 is subjected to diffused reflection on thediffused reflection layer 11 and also subjected to diffusion on thediffusing surface 10 before being transmitted to the wholelight-conducting plate 8, and almost completely uniformed light isconducted from the diffusing surface 10 of the light-conducting plate 8to the whole light-receiving culture surface 4 of the culture bath 3.

To a light-conducting plate 8 described above, a back light technologyused in a liquid crystal display can be applied. In a liquid crystalback light, for example, the upper end portion of a transparent acrylicboard having almost the same shape as the screen of the display servesas a light-receiving portion, and in order for the back of the acrylicboard to function as a diffused reflecting plane, small disks oftranslucent film of which diameter and space are devised are arranged onthe entire back portion by means of the printing process or the like anda sheet of white paper is pasted on that portion. Further, a sheet ofthe same white paper as above is also pasted on the both side portionsand lower end portion of the transparent acrylic board, and on thesurface of the white paper a light-diffusing sheet is pasted in order toobtain a uniform brightness on the screen (for example, applicable istechnology described in Japanese Patent Application Laid-Open No.3-9306, Japanese Patent Application Laid-Open No. 6-317796, JapaneseUtility Model Application Laid-Open No. 6-69903, and Japanese PatentApplication Laid-Open No. 5-34687). As described above, a back lightused in a liquid crystal display may be applied to the light-conductingplate 8 of the present invention; however, uniformity of light which aliquid crystal back light requires is not required for thelight-conducting means of the present invention. And as for alight-diffusing sheet, since it absorbs light, it is more advisable touse a light-conducting plate without a light-diffusing sheet to avoidloss of efficiency.

Experimentally, the efficiency of diffusing and transmitting theincident light from one end of the light-conducting plate to the wholesurface of the light-conducting plate is about 90%. Photosynthesis wasperformed using natural sunlight in the culture bath of the presentinvention utilizing the above technology. The relationships to energywill be discussed below.

On the basis of the quantum theory, at the wave length of 680 nm whichis optimum for photosynthesis, the number of photons per 1 kWh is: 1kWh=1.2×10²⁶ photons, and 1 mol×photon=0.17 MJ. Based on the theoreticallimit of photosynthesis, 1 molecular of CO₂ is fixed by 8 photons andenergy required for fixing 1 molecular of CO₂ is: 8 mol×photon=0.38kWh=1.36 MJ, while the maximum of the experimental results ofphotosynthesis is, for example, 9 mol×photon=0.43 kWh=1.53 MJ.

As for solar energy, the average incident solar energy in the latitudeof Japan is 1 kW/m²; and if solar radiation lasts 4 hours a day, theenergy will become: 4 kWh/m² ·day=14 MJ/m²·day. From this, CO₂ fixingability is: 4 kWh/m² ·day/0.43 kWh/0.044 (kg—CO₂)=0.41 (kg—CO₂/m²·day)=0.1 (kg—CO₂/kWh ·day), when using the maximum of the experimentalresults of photosynthesis.

In the case of an LNG power plant, if the plant operates 24 hours a day,the generated energy per day is: 1.2×10⁷ kWh/day, and at the same timeCO₂ emission per day is 5000 (t—CO₂/day); however, in actuality, CO₂emission per day is below 3200 (t—CO₂/day) (about 64%) consideringworking factor etc.

On the basis of the data so far, the area required for CO₂ fixation is:3200 (t—CO₂/day)/0.41 (kg—CO₂/m² ·day)=8×10⁶m² =800 ha. And solar energyreceived by the area of 800 ha becomes 3.2×10⁷ kWh/day, about 3 to 4times as much as the generated energy, which is a reasonable value interms of theoretical energy balance of photosynthesis.

In the above illustration of FIG. 1, the light-conducting plate 8 andthe culture bath 3 are spaced. However, this photosynthetic culturesystem is more practically effective when more than one systems arearranged in a collective manner. Therefore, a photosynthetic culturesystem is constructed so that the light-diffusing surface of thelight-conducting plate 8 will be in contact with the light-receivingculture surface of the culture bath 3. Alternatively, this type ofphotosynthetic culture system may be constructed so that the side wallof the culture bath 3 forming a light-receiving culture surface willalso serve as a light-conducting plate 8. As for the externallight-receiving surface 13 of the light-converging portion 12, its shapeand size are the same as those of the synthesis of the top surfaces ofboth the culture bath 3 and the light-conducting plate 8 existing underit.

FIG. 3 is a schematic diagram illustrating the construction of acollective photosynthetic culture system 20 in which more than onephotosynthetic culture system 16, which is a unit culture bath of FIG.2, are arranged in a laminated manner and the culture baths areconnected to one another with a connecting pipe for supplying an fluid17, a connecting pipe for transferring product 18 and a connecting pipefor supplying carbon dioxide 19. In FIG. 3, if the shape of thelight-receiving culture surface 13 of a unit photosynthetic culturesystem 16 is rectangular and its long and short sides are W and D,respectively, the width of the external light-receiving surface of theculture bath 3 is allowed to correspond to W and the sum of thethickness of the culture bath and that of the light-conducting plate 8is allowed to correspond to D (see FIG. 2). Accordingly, the shape andsize of the external light-receiving surface of the whole collectivephotosynthetic culture system 20 are the same as those of the topsurface of the laminated culture baths and light-conducting plates underthe external light-receiving surface, which allow light from outside toenter effectively. Construction of a collective photosynthetic culturesystem by laminating the required number of unit photosynthetic culturesystems of the present embodiment in the transverse direction makes itpossible to increase performance capacity of the system while keepingits energy balance.

In cases where plant microorganisms supplied to the culture bath are,for example, Euglena gracilis, when the number of cells in the culturebath reaches of the order of 1 million per 1 ml, the light transmissiondistance is decreased to about 1 cm. This means that the volumetricefficiency is good when the thickness of the culture is about 1 to 5 cm.

As a highly efficient culture bath using natural sunlight, the area ofthe light-receiving culture surface is, for example, set at a valueabout 10 times as large as that of the external light-receiving surface.When the external light-receiving surface is irradiated with the maximumillumination of about 120000 luces of the sunlight, it diffuses thesunlight and the light-receiving culture surface is irradiated withabout 3000 to 10000 luces (70-200 μmol/m²/s) which is suitable forcultivating plant microorganisms (Euglena gracilis). Thus optical energycan be effectively utilized.

When the thickness D of the unit photosynthetic culture system in thisexample is set at about 3 cm and the height H of the same is set at 30cm to 1 m, the calculated value of the carbon dioxide fixing ability of0.2-0.5 kg/m³ is obtained. On the other hand, for conventional culturebaths without a light-conducting means, the carbon dioxide fixingability per unit volume may be capable of being decreased since toointensive light inhibits photosynthesis and light cannot be effectivelyused in the depths of such baths. Consequently, it is expected that theculture baths using light-conducting means shown in the presentembodiment has about 10 times as high volumetric efficiency as that ofthe culture baths without a light-conducting means to which light isirradiated only from above, even when they have the same shape and size.

When the height H of the photosynthetic culture system according to thepresent embodiment is set at 1 m, since the actual situation of carbondioxide emission from a thermal power plant of 500000 kW power is of theorder of 3200 t per day, the area of the system required for carbondioxide fixation is about 800 ha.

Embodiment 2

FIG. 4 is a diagram illustrating the construction of a culture bath anda light-conducting portion of a photosynthetic culture system madepursuant to Embodiment 2 of the present invention. Specifically, similarto the construction shown in FIGS. 1 and 2 described above, aphotosynthetic culture system made pursuant to Embodiment 2 of thepresent invention basically comprises: a culture bath 3 which holds anfluid containing plant microorganisms and whose side consists oflight-receiving culture surface 4; a condenser 22 as light-receivingmeans whose top has a convex external light-receiving surface 23 whichreceives external light 14; a light-conducting plate 8 in the form of aflat plate which turns incident light from the condenser 22 right anglesand reflects light 15 to a light-receiving culture surface 4 of theabove culture bath 3. In accordance with the foregoing embodiment 1, anexternal light-receiving surface of the light-converging portion isflat; however, in accordance with the present embodiment 2, a condenser23 is used as an external light-receiving surface. And in accordancewith the present embodiment 2, there is provided a supporting portion 21on the surface opposite to the light-receiving culture surface 4 of theculture bath 3; however, if the side wall of the culture bath 3 isstrong enough, the supporting portion may be unnecessary. Although notshown in FIG. 4, like the foregoing embodiment 1, carbondioxide-supplying means, a connecting pipe for supplying an fluid andconnecting pipe for transferring product are connected to the culturebath 3.

In FIG. 4, td indicates the thickness of the light-conducting plate 8,tb the thickness of the culture bath 3, ts the thickness of thesupporting portion 21, and tl the width of the condenser 22. W indicatesthe width of the culture bath 3 and it is almost the same as the widthof the light-conducting plate 8 and the length of the condenser 22. Hindicates the depth of the culture bath 3 and it is almost the same asthe height of the light-conducting plate 8. f indicates the focaldistance of the condenser 22 and R the radius of curvature of the convexsurface of the same. In this case, the efficiency of introducing inputlight 24 which enters the light-conducting plate 8 to diffused light 15is 90% or more. One example of a construction of this typephotosynthetic culture system will be shown below. Light entering thelight-receiving culture surface 4, that is, $\begin{matrix}{{{Diffused}\quad {light}\quad 15} = \quad {{input}{\quad \quad}{light}\quad 24 \times \left( {{td}/H} \right)}} \\{= \quad {{sunlight}\quad 14 \times \left( {{tl}/{td}} \right) \times \left( {{td}/H} \right)}} \\{= \quad {{sunlight}\quad 14 \times \left( {{tl}/H} \right)}}\end{matrix}$

and the width of the condenser tl=tb+ts+td=2r

When r=25 mm, the radius R=30 mm, the focal distance=50 mm,

R/r=1.2, f/r=2, and the width across corners of the light-diffusingsurface=13″=275 mm×209 mm

FIG. 5(a) illustrates a state in which the light-receiving culturesurface 4 of the culture bath 3 and the light-diffusing surface 10 ofthe light-conducting plate 8 shown in FIG. 4 are closely touched to eachother, and that is a state when they are actually used. In theconstruction shown in FIG. 5(a), the sunlight 14 outside enters throughthe external light-receiving surface 23 and is converged by thecondenser 22, and the input light 24 enters the light-conducting plate 8through the light intake 9. The light having entered thelight-conducting plate 8 is transmitted to the whole light-conductingplate 8, and almost completely uniformed diffused light 15 is conductedfrom the light-diffusing surface 10 of the light-conducting plate 8 tothe whole light-receiving culture surface 4 of the culture bath 3.

In the present embodiment 2, this type of photosynthetic culture systemmay be constructed so that the side wall of the culture bath 3 forming alight-receiving culture surface 4 will also serve as a light-conductingplate 8. The photosynthetic culture system is constructed so that theshape and size of the projection surface of the external light-receivingsurface 23 of the condenser 22 from above will be the same as thesynthesis of the top surfaces of both the culture bath 3 and thelight-conducting plate 8 which are under the external light-receivingsurface 23. If the condenser 22 is a semicylindrical lens 31 shown inFIG. 5(b), not only it will bring the same good results, but also itwill be very convenient when more than one lens are collectivelyarranged, since semicylindrical lenses are easy to manufacture and theycan be installed to be closely touched to one another on their sides. Inthis case, though the lens differs from the condenser 22 shown in FIG.5(a) in shape, it also condenses incident light from outside in thelight-conducting plate like the condenser 22 shown in FIG. 5(a), asshown with dotted lines. Materials for the condenser may be those havinga high light transmittance such as acrylic and glass, and they can beselected in terms of cost, performance, machinability etc.

FIG. 6 is a schematic diagram illustrating the construction of acollective photosynthetic culture system in which more than onephotosynthetic culture system 26, as a unit, are arranged in a laminatedmanner and their culture baths are connected to one another with aconnecting pipe for supplying fluid, a connecting pipe for transferringproduct and a connecting pipe for supplying carbon dioxide which are notshown in the figure. In the construction shown in FIG. 6, a supportingportion is omitted and, light-receiving means is made up of anintegrated lens 32 which is integrally formed from semicylindricallenses by pressing or the like. Such a construction allows to increasethe output efficiency and reduce production cost. It goes without sayingthat light-receiving means may be made up by arranging more than oneindividual semicylindrical lenses 31 shown in FIG. 5(b) side by side.

In FIG. 6, if the area of the sunlight-receiving surface=the width ofthe condenser (tl)×the length of the condenser (W),the volume of theculture bath=the width of the condenser (tl)×the length of the condenser(W)×the height of the culture bath (H),

the sunlight diffusivity=the height of the culture bath (H)/the width ofthe condenser (tl), and

the illuminance of the surface of the culture bath=the illuminance ofthe surface vertical to sunlight/the sunlight diffusivity,

the sunlight diffusivity is of the order of 12, provided that therequired illuminance of the surface of the culture bath is 10000 luces(135 μmol/sm²) and the average illuminance of the surface vertical tosunlight is 120000 luces. Accordingly, in the construction according tothe present embodiment, if the thickness of the culture bath tb isalmost equal to the width of the condenser tl and the thickness of theculture bath tb is 3-10 cm, the depth of the culture bath H is about 1 mat the most. This is true of the foregoing embodiment 1. Use of aculture bath deeper than 1 m leads to an insufficient supply of light,and even when light is conducted to the depths with optical fibers orthe like, there remains a problem of energy balance.

As for a light-conducting plate 8, in cases where light is conductedfrom one end of the light-conducting plate only and the light is to beconducted to the whole diffusing surface uniformly, the size of thelight-conducting plate is suitably 13″-17″. When the depth is about 30cm, it is suitably about 17″(41 cm×32 cm), wherein the thickness of theculture bath is calculated from the sunlight diffusivity to be about 3cm.

The amount of fixed CO₂ will be described below.

The amount of energy taken in by a unit culture bath depends on the areaof the light-receiving surface and is represented by 4tl·W (kWh). Theamount of CO₂ fixed with this amount of energy is then represented by 0.4tl·W (kg), wherein the depth of the culture bath H is optional as longas it is suitable for culture (provided the unit of tl and w is the m).Accordingly, when using n unit culture baths, the total amount of fixedCO₂ is 0. 4n·tl·W (kg).

In the above embodiments, the system of the present invention was allillustrated by taking the case where light irradiating the externallight-receiving surface is natural sunlight; however, light is notlimited to natural sunlight, but artificial light of high efficiency andluminance, for example, fluorescent light, LED and HID lamp may be usedto irradiate the external light-receiving surface. In such a case,combination of artificial light with natural sunlight according to theweather, time, etc. makes more efficient carbon dioxide fixation byphotosynthesis.

Further, in the above embodiments, the system of the present inventionwas all illustrated to have the construction in which incident light isdiffused and reflected to one surface of the light-conducting plateonly; alternatively, the system of the present invention may have aconstruction in which, for example, as shown in FIG. 7, incident lightis diffused and reflected to both surfaces of the light-conductingplate, that is, a construction in which a light-conducting plate 28 isused which is provided with a diffusion layer on both of the twosurfaces opposite to each other and incident light is conducted to twoculture baths 3 adjacent to the both surfaces. In such a case, sincelight enters a culture bath 3 from its both sides, it is possible toincrease the thickness of the culture bath 3, which means that acollective photosynthetic culture system having the same volume can bemade up of a decreased number of unit culture baths.

Further, in the above embodiments, the system of the present inventionwas all illustrated to have the construction in which the surface of thelight-receiving means (the light-converging portion of Embodiment 1 andthe condenser of Embodiment 2) is not subjected to surface treatment;however, the present invention is not limited to this, one surface orboth surfaces of the light-receiving means may be subjected toprotective coating with a thin film such as UV protective coat. Thismakes it possible to protect each part of the system from deteriorationby UV and mechanical damage, in addition, this is effective in dustprotection.

In the above Embodiment 2, the system of the present invention wasillustrated to have the construction in which the convex surface of thecondenser is the external light-receiving surface; on the contrary, asshown in FIG. 8, the system of the present invention may have theconstruction in which the convex surface of the condenser is turneddownwardly toward the light-conducting plate 8. In such a case, therearises a small gap between the condenser 32 and the light-conductingplate 8, however the size is too small to be a problem. Thisconstruction has the advantage such that, since the top surface is flat,dust is hard to accumulate and it is easy to clean. In FIG. 8, thesystem is constructed so that each light-conducting plate 8 serves as aside wall of each culture bath 3 adjacent to its right and its left. Insuch a case, only one side wall is required for the two adjacent culturebaths 3, which makes the construction easier.

Further, in the above embodiments, the system of the present inventionwas all illustrated by taking the case where plant microorganisms areEuglena gracilis; however, plant microorganisms used in the presentinvention are not limited to Euglena gracilis. Any plant microorganismsincluding algae may be used as long as they can fix carbon dioxideeffectively through photosynthesis.

Further, in the above embodiments, the system of the present inventionwas all illustrated by taking the case where sunlight irradiates thesystem from right above. This does not limit the features of the presentinvention at all and can be easily realized by using reflecting mirrorscapable of homing the sun and optical fibers which change the directionof sunlight irradiation. It goes without saying that, in such a case,the concepts of “above”, “side”, etc. are modified properly.

The shape of the culture bath, light-conducting plate, light-receivingmeans is not limited to that described in the above embodiments.

As is apparent from the description so far, a photosynthetic culturesystem of the present invention allows to avoid photosynthetic medialike algae or plant microorganisms getting directly irradiated withintensive natural sunlight, which means a photosynthetic culture systemof the present invention makes it possible to provide photosyntheticmedia with energy in a very effective manner.

A photosynthetic culture system of the present invention allows toconduct light to the whole volume of the culture bath without usingmechanical energies such as circulation of fluid, diffusion andcirculation of bubbles, which means a photosynthetic culture system ofthe present invention makes possible saving energy, saving area andsaving volume, and consequently, highly increased volumetric efficiencyof the culture system.

INDUSTRIAL APPLICABILITY

Thus, while man is faced with a difficulty of increasing carbon dioxideon a global scale, a photosynthetic culture system of the presentinvention provides improvements such as enhancement of carbon dioxidefixing ability of photosynthetic media, minimization of a photosyntheticculture system including light-conducting means, enhancement ofefficiency, increase of controllability toward products ofphotosynthetic reaction; and consequently, a photosynthetic culturesystem of the present invention makes possible utilization of thetechnology of carbon dioxide fixation on a global scale which preventsglobal warming caused by industrial activities.

What is claimed is:
 1. A photosynthetic culture system comprising: aculture bath holding a fluid containing plant microorganisms, and havinga light receiving culture surface, a carbon dioxide supply for supplyingcarbon dioxide to the culture bath, a flat plate conductor disposedoppositely to the light receiving culture surface, a light receivermounted above an edge of the flat plate conductor for conducting lightto the flat plate conductor, and the flat plate conductor having adiffused reflection layer for reflecting the light diffusely and bendingthe light at substantially right angles to conduct the lightsubstantially uniformly to the light receiving culture surface.
 2. Thephotosynthetic culture system of claim 1 wherein the flat plateconductor has opposing surfaces, and the diffused reflection layer isdisposed on one of the opposing surfaces, and another of the opposingsurfaces has another diffused layer for diffusing the light.
 3. Thephotosynthetic culture system of claim 1 wherein the diffused reflectionlayer includes small disks of translucent film, each disk having apredetermined diameter and a predetermined spacing from one another. 4.The photosynthetic culture system according to claim 1, wherein saidlight receiver has a light-converging portion and the area of the lightentrance surface on top of the light-converging portion is larger thanthat of a bottom surface of the light-converging portion through whichthe light converges.
 5. The photosynthetic culture system according toclaim 1, wherein a side wall of said culture bath also serves as saidflat plate conductor.
 6. The photosynthetic culture system according toclaim 1, wherein a top surface of said light receiver includes both topsurfaces of said culture bath and said flat plate conductor and saidlight receiver is positioned on top of both said culture bath and saidflat plate conductor.
 7. The photosynthetic culture system according toclaim 1, wherein a diffusing layer is provided on one surface of saidflat plate conductor which is opposite to or in contact with saidculture bath.
 8. The photosynthetic culture system according to claim 7,wherein another diffused reflection layer is provided which reflectslight diffusely on another surface of said flat plate conductor which isopposite to the one surface provided with said diffusing layer.
 9. Acollective photosynthetic culture system, wherein more than onephotosynthetic culture system according to claim 1 are arranged so thatthe light-receiving culture surfaces of said culture baths will be inparallel to one another and the photosynthetic culture systems areconnected to one another with a connecting pipe for supplying an fluid,a connecting pipe for transferring products, and a connecting pipe forsupplying carbon dioxide.
 10. The collective photosynthetic culturesystem according to claim 9, wherein more than one light receiver iscarried by said photosynthetic culture system and said light receiversare integrally manufactured.
 11. The collective photosynthetic culturesystem according to claim 10, wherein the light entrance surface of saidintegrally manufactured light receivers is flat.
 12. The collectivephotosynthetic culture system according to claim 10, wherein saidintegrally manufactured light receivers has a thin protective filmformed on a light entrance surface.
 13. The collective photosyntheticculture system according to claim 9, wherein said flat plate conductorhas a diffusing layer provided on both opposing surfaces of said flatplate conductor.
 14. The collective photosynthetic culture systemaccording to claim 9, wherein said flat plate conductor includes twoside walls, which are opposite to each other, of said culture bath. 15.A collective photosynthetic culture system comprising: a plurality ofculture baths, each holding a fluid containing plant microorganisms, andeach having a light receiving culture surface arranged parallel to oneanother, a plurality of flat plate conductors, each disposed oppositelyto a respective light receiving culture surface, a plurality of lightreceivers formed integrally and mounted above the plurality of flatplate conductors, said plurality of light receivers having an integralflat top surface for receiving light, and an integral bottom surfaceconfigured as a plurality of convex surfaces for emitting the light, andeach of the plurality of flat plate conductors receiving the light froma respective one of the convex surfaces and bending the light atsubstantially right angles to conduct the light substantially uniformlyto a respective light receiving culture surface.
 16. The culture systemof claim 15 including a space formed between the integrally formedbottom surface and the flat plate conductors.
 17. The culture system ofclaim 15 including the culture baths connected to one another by a pipefor supplying fluid and another pipe for supplying carbon dioxide.
 18. Acollective photosynthetic culture system comprising: a plurality ofculture baths, each holding a fluid containing plant microorganisms, andeach having a light receiving culture surface arranged parallel to oneanother, a plurality of flat plate conductors, each sandwiched betweenlight receiving culture surfaces of adjacent culture baths, a pluralityof light receivers formed integrally and mounted above the plurality offlat plate conductors, each of the plurality of flat plate conductorsreceiving the light from a respective one of the plurality of lightreceivers and bending the light at substantially right angles to conductthe light to oppositely disposed light receiving culture surfaces, andeach of the plurality of flat plate conductors having opposing surfaces,each opposing surface including a diffusing layer for substantiallyuniformly conducting the light to an opposing light receiving culturesurface.
 19. The culture system of claim 18 wherein the plurality oflight receivers have an integral top surface configured as a pluralityof convex surfaces for receiving light, and an integral bottom flatsurface for emitting the light, and each of the plurality of flat plateconductors receiving the light from a respective one of the convexsurfaces.